1. Field of the Invention
This invention relates in general to an apparatus which records images by jetting a liquid imaging material in a controlled manner. More particularly, this invention relates to an apparatus for depositing ink on a receiving surface by electrostatic generation of intermittent jetting of the ink in response to a signal.
2. Description of the Prior Art
In the past, there have been numerous attempts to effect non-impact printing by positioning a conductive platen behind a sheet of recording media such as paper, and then generating a jet of ink in front of the paper and attracting the ink to the platen by an electrostatic field, thereby attracting the ink to the paper. Examples of such prior art techniques may be found in U.S. Pat. Nos. 3,060,429 and 3,341,859, and in pendng U.S. patent application Ser. No. 334,840, now U.S. Pat. No. 4,404,573 filed Dec. 28, 1981 for An Electrostatic Ink Jet System and assigned to the same assignee as the present application.
In one type of prior art system, a rotating drum serves as the platen, the paper being mounted on the outside of the drum. An ink jet nozzle is positioned a distance from the drum's surface. The drum is held at a reference ground level and a potential is applied to the ink jet to create an electric field which draws ink from the jet nozzle onto the paper.
The latter described system's potential difference is subject to two factors which disturb the electric field necessary to draw the ink to the paper. The first factor is inherent drum eccentricity and wear of the system's mechanical parts which may cause the distance between the drum surface and jet to change. Such a distance change can cause the electric field between the jet and the drum to vary as the drum rotates. Since the maintenance of consistent copy quality requires a constant electric field, these systems are subject to changing print quality as the components wear.
Problems in maintaining a constant electric field are also experienced due to wear in prior art systems employing a flat bed platen.
The electric field created between the platen and ink jet is also subject to variation due to charges which build up on the printing medium or paper. In general, paper is not a good conductor of electricity. Hence, when a charge is induced or delivered to the paper, its potential is raised. Since the paper serves as a reference (ground) voltage, a change in its potential will affect the electric field created when a potential is applied to the ink jet or ink.
The paper's potential can be changed in two undesirable ways. First, static (triboelectric) charges of 500-1000 volts can be induced on the paper due to natural causes such as environment. As the voltage signal applied to the jet nozzle is typically 2000 volts with a 1000 volt data signal added, the static charges can totally overcome the data signal resulting in a condition known as fading.
Second, the paper's potential may be changed as a result of depositing ink on the paper. The delivery of ink is caused by charges within the ink being accelerated by the electric field. These charges are delivered to the paper along with the ink. The paper's poor conducting characteristics do not allow the paper to discharge. As a result, these charges cause the reference voltage to increase, this change in reference voltage changing the electric field created when the potential difference is applied between the platen and jet nozzle. Again, this change in electric field can result in fading.
It is the general object of the present invention to overcome these and other drawbacks of the prior art by providing an electrostatic ink jet printer which delivers a jet of ink from an ink jet nozzle to a printing surface in a controlled manner.
It is another object of the present invention to provide an electrostatic ink jet printer which minimizes the effect of stray or unwanted electrostatic charges on the platen side of the printer.
It is still another object of the present invention to provide an electrostatic ink jet printer which includes a potential barrier which blocks static charges on the printing medium from affecting the electric field created by the application of a potential difference between the printer's ink jet and platen.
It is a further object of the present invention to provide a low cost ink jet electrostatic printer which requires the application of a potential only to the printer's ink jet nozzle.
These and other objects and advantages of the present invention will become more apparent from reading the following detailed description of the invention in conjunction with the drawings provided.